The present invention relates to cable television measurements, and more particularly to in-service cable television measurements of composite triple beat and carrier-to-noise ratios that present no disruption to a viewer of the channel being measured.
Cable television (CATV) systems may be broken down into two major blocks: a headend and a distribution system. The headend is where the television signals are acquired and prepared to be routed to a customer or viewer, and the distribution system carries the prepared signals to the customer's receiver. The signals are sent in a frequency domain multiplexed analog format over a coaxial cable, i.e., each signal is converted to a unique frequency for transmission, using the same transmissions standards as used in broadcast television. To receive a given signal on the cable, the customer tunes the receiver to the signal frequency the same as for a broadcast signal.
The signals in the coaxial cable have to be amplified approximately every two thousand feet or so along the cable to make up for the cable's attenuation or loss. CATV amplifiers are very carefully designed to have low distortion and noise. However every amplifier adds a little extra noise and distortion to the signal the cable carries. The effect of the distortion is to generate new signals, called intermodulation products. These products are called "beats" in the CATV industry. One of the most troublesome of these beats is caused by mixing together three different signals, and is called "triple beats."
Since the picture carrier frequencies are the largest signals in the CATV system and since most of the picture carrier frequencies are on a regular six megahertz spacing, the majority of the triple beat products fall very close to the picture carrier frequencies. In a five hundred megahertz system well over one thousand beats occur near each midband picture carrier frequency. Because there are so many beats, no attempt is made to measure them individually. Instead the total of all the beats, or their composite, is measured.
To minimize the effects of noise, the amplitude of the signals in the system should be maximized. However large signal amplitudes cause the amplitude of the triple beats to increase sharply which makes the television signal muddy. To minimize the effects of triple beats the amplitude of the signals should be minimized, lowering the amplitude of the signals with respect to the noise, making the television signals snowy. To get good performance from a CATV distribution system the amplitude needs to be carefully adjusted to balance distortion and noise.
Previous methods for measuring either noise or distortion require interrupting the service on at least one CATV channel. Due to customer complaints about such loss of service, CATV system operators are reluctant to make these measurements routinely. To minimize these complaints tests are made early in the morning, increasing costs. Two people are required to perform either test: one at the headend to remove the signal and the other in the field to make the actual measurement.
What is desired is a more cost effective method of making these cable television measurements without disrupting service to the customer.